The photooxidation of tryptophan using near-ultraviolet light produces a substance which, in addition to being mutagenic, displays a unique, high level of cell-lethality to recombinationless bacterial mutants and which now has been shown to be hydrogen peroxide by chromatographic, chemical, and biological properties with authentic hydrogen peroxide. To help assess the biological importance of this photolytic process, the mechanism by which hydrogen peroxide is formed from tryptophan is being examined; initial work has implicated the natural tryptophan metabolite, N-formylkynurenine, as a substance which plays a role in promoting this process. A general survey of the breadth of scope of the photo-promoted formation of hydrogen provided with concomitant oxidation of some organic species also is being carried out. Near-ultraviolet light, together with a photo-promoter such as N-formylkynurenine, has been shown to cause peroxide formation from some amino acids and the protein, lysozyme.